Various approaches have been proposed for measuring exhaust gas temperature for engine control, such as using temperature estimates and temperature sensors. However, to reduce excess sensors, an alternative approach to temperature sensing uses heater exhaust gas oxygen sensors. For example, a heater of an exhaust gas oxygen sensor may be disabled and used as an indication of temperature.
However, the inventors herein have recognized various issues with such an approach. First, because the heater is disabled, the sensor may unavailable for use in air-fuel ratio control, thus leading to degraded engine operation when used for temperature measurement. In addition, it may be advantageous to sense temperature in locations where it is unnecessary, or merely superfluous, to also sensor oxygen content.
To address the above competing interests and contradictory approaches, in one embodiment, a system is provided for an internal combustion engine of a vehicle, comprising an emission control device in an exhaust stream of the engine, an exhaust gas oxygen sensor coupled upstream of the emission control device, a temperature sensor coupled adjacent the emission control device, and a controller coupled to the exhaust gas oxygen sensor and the temperature sensor, the controller identifying degradation of the temperature sensor responsive to the exhaust gas oxygen sensor during lean operation, where the controller operates the exhaust gas oxygen sensor in a temperature sensing mode to identify the degradation of the temperature sensor. In this way, it is possible to take advantage of a temperature sensor in the exhaust, while also enabling diagnostics for the temperature sensor via the exhaust gas oxygen sensor. In particular, because the exhaust gas oxygen sensor is used to diagnose degradation of the temperature sensor during lean operation (where the oxygen signal information may not be used), accurate air-fuel ratio control about stoichiometry can be maintained. Further, even though the temperature sensor is located in a caustic environment experiencing high temperatures, it is still possible to provide diagnostics without additional exhaust temperature sensors (although such sensors may be used, if desired).